1. Field of the Invention
The present invention relates to technologies pertaining to polishing of silicon carbide crystals, and particularly to crystal polishing technologies pertaining to rough polishing.
2. Description of Related Art
Power devices that handle electric energy with low loss are used widely since they achieve a significant reduction in power consumption. Currently available power devices are manufactured from silicon substrates. However, due to the inherent characteristics of silicon, there is a limit to the further enhancement of the performance of these devices. In particular, since silicon cannot be used at high temperatures, there is a need for a material to replace silicon.
For this reason, silicon carbide (SiC) recently has been receiving attention. The width of the forbidden band of silicon carbide is three times wider than the width of the forbidden band of silicon, so that silicon carbide can be used at a higher temperature than silicon. The dielectric strength of silicon carbide is about ten times greater than that of silicon, and therefore, further miniaturization of power devices can be achieved through the use of silicon carbide. Moreover, the thermal conductivity of SiC is about three times higher than that of silicon, so that silicon carbide also has an advantage in that it has excellent heat dissipation and is easier to cool. Since silicon carbide has excellent characteristics as described above, silicon carbide substrates are promising substrates to replace silicon substrates for power devices.
In order to produce power devices from silicon carbide substrates, it is ultimately necessary to polish the surface of the substrates to be as smooth as possible. At present, a silicon carbide ingot that has been manufactured by crystal growth is sliced first, for example, by a wire saw or a blade saw. Then, grinding is performed to remove any irregularities present in the substrate, and thereafter, rough polishing using a hard polishing surface plate and precision (mirror surface) polishing using a soft polishing pad are performed. Of these steps, precision polishing is the most important step, and known techniques, such as the polishing technique disclosed in JP 2001-508597A, that use a suspension containing abrasive diamond particles or SiO2 (colloidal silica) commonly are used for precision polishing at present. Similarly, rough polishing, which is a pre-treatment step for a final processed surface, is a step that is as important as precision polishing. If the substrate is in good state after rough polishing, then the processing time may be expected to be reduced in the final step.
However, since the technique disclosed in JP 2001-508597A uses diamond as abrasive particles, a significant amount of damage is caused to the objects to be polished, although the time required for polishing (processing rate) is short. That is, during polishing, great damage is caused to silicon carbide crystals, and flocs that are produced from the abrasive diamond particles tend to cause damage to silicon carbide crystals. Furthermore, the rough polishing step uses a hard polishing surface plate. Therefore, a significant amount of damage also is caused to the polishing surface plate, and it is necessary to correct the surface of the polishing surface plate during the subsequent polishing. In rough polishing using a hard polishing surface plate, the shape of the polishing surface plate is transferred onto the silicon carbide substrate, which is the material to be polished. Accordingly, when rough polishing is carried out with a polishing surface plate whose surface has not been corrected, the state of the processed surface of the silicon carbide substrate may deteriorate. Therefore, it is necessary to reduce the damage caused to the polishing surface plate when polishing silicon carbide crystals.